The field of the invention is liquid fertilizers produced from concentrated wet-process phosphoric acid. The invention is particularly concerned with the production of sludge-free liquid fertilizers from merchant grade phosphoric acid.
Merchant phosphoric acid is produced from phosphoric acid by the so-called "wet process" in which phosphate rock is digested with sulfuric acid. The initial phosphoric acid product may contain from 23 to 33% P205. The crude phosphoric acid, after filtration, is concentrated by evaporation of water to obtain commercial grade phosphoric acid. The merchant grade acid usually contains around 54% P205, while the superphosphoric acid contains about 70% P205.
Concentrated phosphoric acids, such as merchant or superphosphoric, can be reacted with ammonia to produce ammonium polyphosphate reaction products, which may be used in liquid or solid fertilizers. A commercial conversion process is described in U.S. Pat. No. 3,464,808. A related process has been developed by the Tennessee Valley Authority. See Melin, et al., Farm Chemicals, 133 (11): 26-36 (1970).
There has been a long-recognized and heretofore not adequately solved problem of how to produce stable liquid fertilizers from concentrated wet process phosphoric acids. Phosphate rock contains metal impurities which are solubilized curing the digestion of rock and are present in the crude acid. Such impurities include iron, aluminum, magnesium, calcium, sodium, and potassium.
Heretofore the iron and aluminum impurities have been particularly troublesome. Frazier et al. (1966) cited below point out that iron and aluminum salts are quite soluble in merchant grade acid. However, upon ammoniation the solubility of Fe.sup.+3 and Al.sup.+3 decreases rapidly. Resulting precipitates of iron and aluminum compounds create a handling problem and also immobilize phosphorus in forms not readily available to crops. When ammoniation is to the usual neutral pH range of 5 to 7, gelatinous iron/aluminum precipitates can form. Such precipitation, often referred to as "sludging," can continue during storage of the liquid fertilizer. Shelf life for ammonium polyphosphate liquid fertilizer in excess of six months is desired by the fertilizer industry, but has not heretofore been satisfactorily obtained with liquid fertilizer prepared from merchant grade acid.
With the mining and use of phosphate rock containing increased amounts of magnesium, it has become desirable to remove magnesium from the crude phosphoric acid. Ion exchange treatment can be employed for Mg removal, as described in U.S. Pat. Nos. 4,385,993, 4,363,880, and 4,493,907. While cation exchange resins are effective for removing magnesium and calcium, such treatment results in very little reduction in trivalent aluminum (Al.sup.+3) and ferric iron (Fe.sup.+3). Divalent iron (Fe.sup.+2) can be removed but in phosphate rock most of the iron is present in trivalent form. Cation exchange also achieves some reduction of monovalent metals (e.g., Na.sup.+ and K.sup.+).
Concentration of wet process phosphoric acid by evaporation of water beyond 60-62% P205, depending upon the impurity content, converts the orthophosphates to polyphosphates. Increased concentration produces additional polyphosphates. This is desirable since pyrophosphate and other polyphosphates are capable of sequestering metal ions, and thereby can contribute to the stability of the concentrated acid and to liquid fertilizers prepared therefrom. Heretofore, however, it has not been possible to produce ammonium polyphosphate liquid fertilizers directly from merchant acid which remains stable and sludge-free on prolonged storage. The following references recognize this problem: U.S. Pat. No. 3,988,114; Frazier, et al., J. Arg. Food Chem. (1966), 14: 522-529; and Frazier, et al., "Stabilizing Liquid Fertilizer with Fluorine Compounds," in Fertilizer Solutions, Issue July-August 1972.
The industry's answer to the above problem has been to utilize superphosphoric acid for the production of ammonium polyphosphate liquid fertilizers. The increased sequestering capacity of the higher polyphosphates present in the superphosphoric acid can provide improved stability. However, with extended holding of the liquid fertilizers, sludging problems are still encountered. This has led to proposals for the addition of stabilizing compounds to promote sequestration and/or suspension of the metal compounds. See, for example, Frazier et al. (1972), cited above, and U.S. Pat. No. 3,988,140.
It has also been proposed to reduce the concentrations of iron and aluminum compounds by blending merchant and/or superphosphoric acids with wet process phosphoric acid which has been subjected to solvent extraction to remove metal ions. Such a procedure is described in the U.S. Pat. No. 3,988,140. Similarly, furnace process phosphoric acid, which is very low in metal ions, can be blended with wet-process phosphoric acid to reduce metal ion concentrations. These procedures, however, add substantially to the cost of phosphoric acid as a raw material for producing liquid fertilizers.
The foregoing problems have led to intensive study of the complex precipitated impurities for wet-process phosphoric acid. See, for example, Lehr, et al., J. Agr. Food Chem. (1966) 14: 27-33. The search has continued for a practical, low-cost process for producing sludge-free liquid ammonium polyphosphate fertilizers from merchant phosphoric acid.